The present invention relates to light modulation devices incorporating electrostatically orientable flaps and relates particularly to such devices having matrix addressing.
A light modulation device with microflaps has already been disclosed in Swiss Patent No. 633 902, granted on Dec. 31, 1982. This device, produced on a silicon substrate, incorporates a plurality of microflaps capable of being rotated under the effect of an electrical field applied between the electrodes situated on both sides of the flaps. In the absence of an electrical field, the flaps are held at rest in the plane of the substrate by means of flexible attachments fixed to the substrate. The device, as disclosed, incorporates no means permitting matrix control of the flaps; that is to say the control of the flaps, arranged in a matrix, through row electrodes and column electrodes. On the contrary the flaps of the disclosed device each require their own control electrodes.
FIGS. 1.a and 1.b show the device as disclosed in Swiss Patent No. 633 902. Flaps V1 to V4 are able to rotate above a cavity 3, produced in a substrate 5, when they are subjected to an electrical field E. The electrical field is created by the application of a voltage between a common electrode 6 (FIG. 1.b), placed on a plate of glass 8, and the electrode 7 placed underneath the flap. When the voltage between these two electrodes is cancelled, the flap which was addressed returns to its position of rest, that is to say parallel to the plane of the substrate, under the action of the flexible attachments 4 which attach it to the substrate. It is clear that, in the application described, the control of the flaps cannot be of the matrix type.
Swiss Patent No. 641 315, granted on Aug. 31, 1984 under the title "Miniature Display Device With Microflaps", discloses a means of achieving matrix control of the microflaps by associating them in pairs and applying between the flaps of the same pair, when they are addressed, a voltage suitable for maintaining them in this state even when the main electrical control field disappears. In order to avoid the flaps from "sticking" when they are thus held, it is necessary to produce insulating structures on the flaps themselves or again to provide stop elements preventing the flaps from coming into contact. It will easily be understood that such means are not easy to implement and solutions have been sought allowing matrix control of the flaps without it being necessary to have recourse to any modifications of the flaps or their environment.
The device shown in FIG. 2, which is the subject of the above mentioned Swiss Patent No. 641 315, is arranged so as to permit matrix control of the flaps. To that end the flaps V are arranged in pairs and can be maintained in an active state, that is to say in an orthogonal position with respect to the plane of the substrate 1, through the application of a sufficient voltage between the flaps in the same pair. For this purpose, the flaps incorporate at least one conductive layer connected to a maintenance electrode a1, b1, a2 or b2. The electrodes c1 to c4, in conjunction with the electrodes ai and bi, serve to create the electrical field for addressing the flaps. As indicated previously, it is necessary to prevent the conductive layers on the flaps from being able to come into contact with each other when the flaps are activated; which requires the use of special stop means described in the said patent.